The present invention relates to a heat resistant catalyst for the production of sulfuric acid which can be used especially for a high SO.sub.2 concentration gas and can be maintained oxidation activity at a high temperature for a long period of time in addition to oxidation activity at a usual low temperature in a catalytic oxidation of sulfur dioxide (SO.sub.2) to sulfur trioxide (SO.sub.3). More particularly, it relates to a heat resistant catalyst composition which is suitable for using a high SO.sub.2 concentration gas maintaining oxidation activity at both low and high temperatures for a long period of time by specifying a composition ratio of vanadium pentoxide and potassium sulfate and the like.
Processes for the catalytic production of sulfuric acid by using vanadium catalysts have been succeeded industrially from old time. Catalytic oxidation reaction yield of SO.sub.2 to SO.sub.3 by these vanadium catalysts was about 95 mol % by approximately 1955, but it became 98 mol % by improvements in the art afterwards. Further double absorption process was introduced in 1962 and the yield became 99.5 mol %. And plants which give 99.9 mol % of yield are under construction now as further improved process. Therefore, unoxidized rest part of SO.sub.2 gas decreased from 5 mol % to 2 mol %, further to 0.5 mol % and to 0.1 mol % in the above described improved cases.
Vanadium catalysts used in these processes, however, are changed little in components and compositions thereof for recent 20 years, so the above described improvement of oxidation yield can be deemed as merely a result of the improvement of reaction system. This fact means that the SO.sub.2 conversion could reach somewhat 95% even if the catalyst currently used in newest reaction process is applied to a conventional single absorption and a catalytic convertor of two catalyst layers.
On the contrary, recently Wellman-Load Process has been developed as one of an exhaust gas desulfurization process to obtain a SO.sub.2 gas in a concentration of 100 mol %, so that an about 25 mol % of SO.sub.2 containing gas which is obtained by mixing air as an oxygen source into the SO.sub.2 gas can be used industrially as a raw material for producing sulfuric acid. That is to say, about three times of a high concentration gas in comparison with a concentration of a conventional raw gas, e.g., a 7 mol % concentration SO.sub.2 gas can be used, so the same amount of sulfuric acid can be produced using one third volume of feed gas, therefore cost of its plant construction has considerably decreased. For this purpose, however, it is required for the vanadium catalyst used for the process to have a property so as to resist a higher reaction temperature than a conventional reaction temperature and maintain its activity for a long period of time in addition to a conventional activity performance.
The reason is that the reaction gas temperature elevates about 2.0.degree. C. per increase of 1.0 mol % of conversion in case of using a gas containing 7.0 mol % of SO.sub.2 and the elevation of temperature is proportional to approximately SO.sub.2 concentration, because the oxidation reaction of sulfur dioxide is exothermic and it is carried out adiabatically in an industrial reactor. Furthermore, if a raw gas containing SO.sub.2 in a concentration of 21 mol % is used, the gas temperature elevates 6.degree. C. per increase of 1 mol % of conversion. In an industrial reactor, the vanadium catalyst is used by packing in it dividing three or four layers, but an inlet gas temperature of the first layer is usually controlled to about 430.degree. C. Thus, if 7.0 mol % of SO.sub.2 containing gas is used as raw material and it is reacted in a 75 mol % of conversion in the first layer as usual, the outlet gas temperature of the first layer becomes 580.degree. C. (=2.times.75+430) on calculation in case of neglecting heat loss in the reaction. However, when 21 mol % of SO.sub.2 containing gas is used, it should be kept in mind that the outlet gas temperature of the first layer becomes high temperature such as 670.degree. C. (=6.times.40+430), even if the conversion in the first layer is controlled down to only 40 mol %.
Judging from former common sense, it has been thought that it is impossible to adopt such high outlet gas temperature of the catalyst layer with respect to heat resistance of a material of the reactor. But good heat resistant metallic materials have been applied recently, so a catalyst which can be a resistant to a high temperature of more than 650.degree. C. and can maintain its activity for a long period of time is required as a catalyst to be charged into the first or second layer of the reactor.
In an oxidation reaction of SO.sub.2 to SO.sub.3, the higher the temperature becomes, the lower an equilibrium conversion which can be reached theoritically becomes in spite of existence of the catalyst. Thus, once the reaction in each catalyst layers reach to the equilibrium at each temperature respectively it cannot proceed no more, even if excess catalyst is packed into the layers. Therefore, a reacted gas approached to this point is cooled to approximately an initial inlet temperature by means of a heat exchanger and then introduced into the next catalyst layer to proceed the reaction.
From the above described facts, even the catalyst which is used for a process using high concentration raw gas containing 21% by volume of SO.sub.2 should have superior activity at a low inlet temperature in a conventional catalyst layer. Especially in case of using a SO.sub.2 gas from Wellman-Load Process which is applied to desulfurization in a power generating plant, the load change at night and day is great and low concentration gas containing less than 7% by volume of SO.sub.2 should be occasionally used based on shortage of the raw gas. Therefore, the activity at the low temperature is required without fail. As a catalyst for the production of sulfuric acid having durable activity at a high temperature, Fe.sub.2 O.sub.3 granular catalyst is well known from old times, but its oxidation activity at temperature of lower than 500.degree. C. is very poor, so it cannot be used for the production of sulfuric acid. Further, although a platinum catalyst has high activity at both high and low temperatures, it cannot be used actually, because it is expensive and its activity decreases by poisoning with arsenium.